The present invention relates to a novel process for extending the life of an antifouling paint film for structures immersed in sea water. More particularly, the invention relates to a process for extending the life of the paint film by applying onto an antifouling paint film a coating of a graft polymer which is capable extending the life of the paint film.
The maintenance of the antifouling property of an antifouling paint film for structures immersed in sea water, for instance, moving structures such as ships and stationary structures such as wharves, piers and bridges, for a long period of time contributes greatly to the economical operation of the structures and the effective utilization of natural resources. Accordingly, there has been a constant demand for the development of a paint having a superior antifouling property and of a process for maintaining the antifouling property of the paint film for a long period of time.
The antifouling paints for preventing the fouling of aquatic organisms are classified into two large groups. One group is those employing a hydrophilic synthetic polymer as a main binder. The other group is those employing rosin as a main binder. In the paint film formed from such a paint containing a rosin binder, an antifouling agent contained in the paint film is leached from the paint film with the leaching of rosin. Most antifouling paints belong to this group. However, the leaching of rosin results in the degradation of the paint film per se. In fact, the paint film formed from a paint of the latter group is considerably eroded and the toughness of the paint film is lowered, when it is immersed in sea water for several months.
U.S. Pat. No. 3,990,381 to Shepherd discloses various polymers employed for coatings, especially antifouling coatings, for watercraft and underwater static structures, e.g. copolymers of hydrophilic monomers such as methyl methacrylate-hydroxyethyl methacrylate copolymers, and copolymers of vinyl pyrrolidone and acrylic monomers. However, these polymers are water-soluble or have a poor water-resistance, and in order to render then water-insoluble or water resistant, they must be cross-linked, for instance, by means of cross-linking agents. The cross-linked antifouling coatings are hard to leach antifouling agents and, therefore, the antifouling property is remarkably decreased.
In general, when an antifouling ship bottom paint film is allowed to stand in air for several weeks, checking or cracking occurs. Also, with the lapse of time the roughness which is accelerated by navigation occurs on the surface of the film. Upon navigation the roughness causes the occurrence of turbulence by which the erosion of the film is remarkably accelerated. Also, the turbulence causes speed down and extra fuel is required to speed up. Although there is no fouling during navigation, a ship bottom is fouled when a ship is at anchor. Therefore, a ship bottom paint is required to have both the film toughness and antifouling property.